Thyristors are commonly used for high current and high voltage applications in motor drive circuits, appliance controls, robotics, lighting ballasts, solid-state relays, power inversion in high-voltage transmission lines and in other high power applications. This is because thyristors exhibit bistable characteristics and can be switched between a high-impedance, low-current OFF state and a low-impedance, high-current ON state. Thyristors are now available with current ratings from a few milliamperes to over 5000 A and voltage ratings extending above 10,000 V. A comprehensive treatment of the operation of thyristors can be found in a textbook entitled Semiconductor Power Devices, by S. K. Ghandhi, John Wiley & Sons, N.Y. (1977).
Semiconductor switching devices such as the diode AC switch (DIAC) of FIG. 1 and the triode AC switch (TRIAC) of FIG. 3 have also been used extensively for high power AC switching applications. This is because they offer thyristor characteristics in two directions and have ON and OFF states for both positive and negative anode voltages. These characteristics are illustrated by FIGS. 2 and 5, which contain the first and third quadrant I-V curves for the DIAC and TRIAC, respectively. As will be understood by those skilled in the art, the TRIAC is essentially two thyristors connected in antiparallel and as illustrated by FIGS. 4a-d, the on-state currents flow in two different areas depending on the quadrant of operation. This implies that for a given current rating, the area of a conventional TRIAC is twice that of a similar unidirectional thyristor. These and other aspects of TRIACs are described in the textbook entitled Physics of Semiconductor Devices, by S. M. Sze, John Wiley & Sons, N.Y., pp. 229-234 (1981). In addition to the above described properties, the TRIAC also requires a moderate amount of gate current to trigger it into conduction in both the first and third quadrant modes of operation and does not exhibit current saturation at high voltages or gate-controlled turn-off capability. Instead, the TRIAC is turned off by reversing the anode potential. These features of the TRIAC cause it to lack both forward-biased safe-operating-area (FBSOA) and reverse-biased safe-operating-area (RBSOA).
Therefore, notwithstanding the extensive use of TRIACs for high-power AC applications, there continues to be for a high power semiconductor switching device which has reduced lateral dimensions, current saturation and low power gate controlled turn-on and turn-off capability in both the first and third quadrants of operation.